Rotating beacons have been a mainstay of emergency service vehicles for many years. In its most fundamental a rotating beacon comprises a continuous light source that is focused into a beam and is either rotating or, more commonly, is located at the focus of a rotating reflector. The focussing element is most usually a parabolic reflector.
A rotating reflector may comprise the focussing element, but need not necessarily be so. For example, a parabolic focussing reflector may direct a beam into a rotating plane mirror. The optical parts are generally housed in an optically transparent dome closed over a base assembly including the motor components necessary to rotate the reflector and to lead in electrical power the motor and lamp.
One example of an incandescent-globe illuminated rotating beacon is that disclosed in DE 4304216 A1 published 18 Aug. 1992. A beacon has a stationary halogen lamp (16) mounted on a bearing bracket (31), which is arranged on a lamp base (11). A vertical light beam is generated by the lamp (16) and parabolic reflector (17), and is turned into a substantially horizontal plane by a motor-driven plane mirror (24). The lamp (16) is mounted from below in the bracket.
The technology is constructed to accommodate the considerable heat generated by the QH globe. With a yield of 24 lumens/Watt and an overall thermodynamic efficiency of 3.5%, a 50 W lamp beams at 1200 lumens while generating 48.25 W of heat. While most of the heat is radiated out with the light, the rotating beacon must deal with heat generated by conduction and convection heating of the beacon components.
With the advent of high intensity LEDs, there are examples of rotating beacons using this solid state technology. Typically, a polymer lower housing mounts an LED assembly including a heat sink, a driver circuit, and an electric motor driving a rotating parabolic reflector. The reflector is housed in a transparent polycarbonate upper housing. While the LEDs are more efficient at 14%, a 30 Watt LED array would generate about 26 Watts of heat, substantially all of which would be retained by the heat sink. In a closed system the heat build-up is such that high intensity LED beacons are not used beyond about 10 Watts. Even at this low power, the housing must be ventilated, exposing the electronics to the environment. The light output of the diodes varies with temperature.
US2012182730A1 discloses a plurality of LEDs of differing colours and disposed in groups circumferentially spaced about an axis. The plurality of LEDs is encompassed by a two-lens optical system comprising a collimating lens and a condensing, coupling lens. The groups are selectively illuminated for a rotating effect or another pattern. This has the disadvantage in a beacon of only applying a fraction of the available intensity at any one time in the momentary direction of sweep.
US2012250312A1-1 discloses a semi-parabolic reflector assembly rotated by a side mounted, belt driven motor assembly. The reflector is stated to have an aperture into which an integral base extension protrudes as a heat sink, the base extension supporting the LEDs in an array centred substantially at the focal point of the semi-parabolic reflector. The belt drive necessitates a conventional electric motor that resists the inherent side loads of the belt, which raises the profile of the apparatus. The reflector bearing must necessarily be large and robust. There is a great deal of clutter in the space available for ventilation. It confines the space available for control elements such as a PCB.